1. Field of the Invention
The invention concerns an illumination system for wavelengths xe2x89xa6193 nm, particularly for EUV lithography, a method for adjusting the illumination in an exit pupil of an illumination system, as well as a projection exposure comprising such an illumination system.
2. Description of the Prior Art
In order to allow even further reduction in the line widths for electronic components, particularly in the submicron range, it is necessary to reduce the wavelength of the light used for the microlithography. For example, with wavelengths less than 193 nm, lithography with soft X-rays, so-called EUV lithography is possible.
An illumination system suitable for EUV lithography should illuminate homogeneously, i.e., uniformly, the field used in EUV lithography, particularly the ring field of an objective, with as few reflections as possible, and furthermore, the pupil of the objective should be illuminated up to a particular filling ratio a, independently of the field, and the exit pupil of the illumination system should lie in the entrance pupil of the objective.
From U.S. Pat. No. 5,339,346 an illumination system for a lithography device that uses EUV radiation has been made known. For uniform illumination in the plane of the reticle and filling of the pupils, U.S. Pat. No. 5,339,346 proposes a condenser, which is constructed as a collector lens and comprises at least four mirror facets arranged in pairs and symmetrically. The light source used is a plasma light source.
In U.S. Pat. No. 5,737,137 there is shown an illumination system with a plasma light source comprising a condenser mirror. In U.S. Pat. No. 5,737,137 an illumination of a mask or reticle is achieved by means of spherical mirrors.
U.S. Pat. No. 5,361,292 shows an illumination system in which a plasma light source is used. The point-like plasma light source is imaged into a ring-shaped illuminated surface by means of a condenser having five aspheric mirrors arranged eccentrically. The ring-shaped illuminated surface is then imaged in the entrance pupil by means of a special sequence of grazing-incidence mirrors.
From U.S. Pat. No. 5,581,605 an illumination system in which a photon emitter is divided into a plurality of secondary light sources by means of a honeycomb condenser is known. In this way, a uniform illumination is achieved in the plane of the reticle. The imaging of the reticle on the wafer to be exposed is done with conventional reduction optics. Exactly one rastered mirror with elements of equal curvature is arranged in the illumination beam path.
EP 0 939, 341 shows a Kxc3x6hler illumination system for wavelengths  less than 200 nm, especially for the EUV range, with a first optical integrator comprising a plurality of first raster elements and a second optical integrator, comprising a plurality of second raster elements. The light distribution in the exit pupil is controlled by a diaphragm wheel, which involves a considerable loss of light. As an alternative to this, for a quadrupole distribution of light, it is proposed to split the light beam after the source and before the first optical integrator into four light beams. Various other illumination settings can also be achieved according to EP 0 939,341 without the use of diaphragms, for example, by changing optics. This type of variation of the illumination settings is on the one hand very costly, and on the other hand limited to certain types of illumination settings, namely, annular and quadrupolar illumination.
From DE 199 03,807 A1 there is another known EUV illumination system. The system according to DE 199 03 807 comprises two mirrors or lenses with raster elements. Such systems are also known as double-faceted EUV illumination systems. The disclosure content of DE 199 03,807 A1 is fully incorporated into the present application.
In DE 199 03,807 A1, the basic construction principle of a double-faceted EUV illumination system is illustrated. The illumination in the exit pupil of the illumination system is determined, according to DE 199 03,807, by the arrangement of the raster elements on the second mirror. A variable controlling of the illumination in the exit pupil or the adjustment of a predetermined distribution of the illumination in the exit pupil is not described by simple means according to DE 199 03,807.
The object of the invention is to indicate the most simple possible construction of a double-faceted illumination system, which allows a variable adjustment of any illumination distribution in the exit pupil with substantially no losses of light, as well as a method for adjusting an illumination distribution in such an illumination system.
According to the invention, the object is solved in that, in an illumination system for wavelengths xe2x89xa6193 nm, particularly for EUV lithography, a predetermined illumination in an exit pupil is adjusted by altering points of incidence of light channels traveling from a light source to the exit pupil. Thanks to such an adjustment of the light distribution in the exit pupil, any given distributions can be realized and losses of light, such as occur for example in the solutions using diaphragms, can be avoided.
While the system is designed to be purely reflective at wavelengths in the EUV range, i.e., designed exclusively with mirror components, refractive components such as lenses or lens arrays are used as so-called optical integrators in 193-nm or 157-nm systems.
Thus, the invention also provides an illumination system in the 193-nm and 157-nm range, with which the illumination of the exit pupil can be adjusted and changed in a simple manner.
With the illumination system of the invention, the field in the plane of the reticle is illuminated homogeneously and with a partially filled aperture. Furthermore, the exit pupil of the illumination system is illuminated variably.
As described below, several light distributions in the exit pupil can be obtained with the help of the invention.
For circular illumination, the distribution of light or the illumination setting in the exit pupil, which in the present case coincides with the objective pupil, is defined by the filling factor "sgr":
Filling factor: "sgr"=rillumination/Robjective aperture
Here, rillumination means the radius of the illumination and Robjective aperture is the radius of the objective aperture.
By definition, for "sgr"=1.0, the objective pupil is completely filled; and, for example, "sgr"=0.6 corresponds to less than complete filling.
For an annular distribution of light, the objective pupil is illuminated in annular fashion. To describe this, one can use the following definition of "sgr"out/"sgr"in:             σ      out        =                                        r            ⁡                          (              90              )                                            R            ⁡                          (                              NA                max                            )                                      ⁢                  xe2x80x83                ⁢        whereby        ⁢                  xe2x80x83                ⁢                              ∫            0                          r              ⁡                              (                90                )                                              ⁢                                    l              ⁡                              (                r                )                                      ⁢            r            ⁢                          ⅆ              r                                          =              0.9        ·                              ∫            0                          R              ⁡                              (                                  NA                  max                                )                                              ⁢                                    l              ⁡                              (                r                )                                      ⁢            r            ⁢                          ⅆ              r                                                      σ      in        =                                        r            ⁡                          (              10              )                                            R            ⁡                          (                              NA                max                            )                                      ⁢                  xe2x80x83                ⁢        whereby        ⁢                  xe2x80x83                ⁢                              ∫            0                          r              ⁡                              (                10                )                                              ⁢                                    l              ⁡                              (                r                )                                      ⁢            r            ⁢                          ⅆ              r                                          =              0.1        ·                              ∫            0                          R              ⁡                              (                                  NA                  max                                )                                              ⁢                                    l              ⁡                              (                r                )                                      ⁢            r            ⁢                          ⅆ              r                                          
Another light distribution is the so-called quadrupole illumination for imaging of xe2x80x9cManhattan structuresxe2x80x9d, for example.
According to the invention, all of the above-described settings can be realized at the same time in double-faceted illumination systems. In a first embodiment of the invention, on the second optical element with raster elements, hereinafter also called pupil raster elements or pupil honeycombs, the distribution of the second raster elements on the second optical element for all possible illuminations in the exit pupil are available.
By replacing the first optical element or lens with raster elements, hereinafter also called field raster elements or field honeycombs, or by changing the tilt of the raster element on the plate of the first optical element, then only the pupil raster elements of a particular setting, such as the quadrupole setting, can be illuminated on the second optical element. To achieve this the pupil raster elements are adapted to the illumination of the field raster elements. The illumination of the field raster elements depends on the light source. Between field raster elements and pupil raster elements by a tilt angle of raster elements or prismatic effect of the raster elements, there occurs a sorting of the input light distribution on the field raster elements into the output light distribution on the pupil raster elements and thus in the entrance pupil of the lithography objective.
In an alternative embodiment of the invention, the arrangement of the pupil raster elements on the second raster element does not provide for all possible illumination settings. The illumination is then adjusted, for example, by replacing both the first and the second optical element. The replacement accomplishes an adjustment of the point of incidence of the light channels in the exit pupil, which are determined by assigning the raster elements of the first optical element to raster elements of the second optical element, and thus the light distribution in the exit pupil is adjusted.
Of course, in an alternative embodiment, the illumination in the exit pupil can be achieved by moving and/or tilting the raster elements of the second optical element and the tilting the raster elements of the first optical element.
Hereinafter, the preferred embodiments of the invention shall be described with exemplary mirror systems, without this entailing a restriction to reflective systems. In reflective systems, the first and second optical elements with raster elements are faceted mirrors. The person skilled in the art will be able to transfer the steps given as examples to refractive systems without inventive activity, and without this being explicitly mentioned.
In the systems according to the invention with two optical elements with raster elements, the form of the pupil raster elements is adapted to the form of the secondary light sources and thus it differs from the form of the first field raster elements. The pupil raster elements are preferably especially elliptical or round, when the light source is also round in shape.
In a preferred embodiment of the invention, the field and pupil raster elements have a prismatic effect, i.e., they deflect the main beam through each individual raster element according to a predetermined angle.
The field raster elements can additionally have either an isotropic optical effect and the same aspect ratio as the field being illuminated, or an anisotropic effect with a smaller aspect ratio than the field being illuminated.
In order that each light bundle of each field raster element overlaps in the field plane, e.g., the reticle plane, in an advantageous embodiment of the invention, the pupil raster elements are inclined or tilted in relation to the pupil raster element plate, which supports the plurality of pupil raster elements.
If the system is constructed as a system with real intermediate images of the light source in the light path after the field raster element plate, the pupil raster elements can serve at the same time as field lenses for the coupled imaging of the light source in the entrance pupil of the lithography objective.
A light bundle that travels from a field raster element of the first mirror, i.e., a field honeycomb, to a pupil raster element of the second mirror, i.e., a pupil honeycomb, is designated as a light channel in the present application. The number N of light channels is determined by the number of field raster elements being illuminated.
In the first configuration of the invention, the number of pupil raster elements M of the pupil raster element plate is always greater than N, since the pupil raster elements for all adjustable illumination settings in the exit pupil are arranged on the pupil raster element plate. Therefore the pupil raster element plate supports more pupil raster elements than would be necessary from the number of channels, which are determined by the number of field raster elements on the field raster element plate. This, in turn, means that there are more pupil raster elements arranged on the pupil raster element plate than channels and only some of the pupil raster elements are illuminated at a setting with a particular number of field raster elements having N channels. This leads to a segmented or parcelled illumination of the pupil. A necessary requirement for this is that the etendue of the light source is smaller than the etendue of the lithography objective. If this is not the case there is no segmented illumination of the exit pupil. Therefore, it would no longer be possible to accommodate more raster elements than channels on the pupil raster element plate in the pupil plane and there would be a loss of light and light scattering by crosstalk among the channels. At present, the etendue of EUV sources, such as synchrotron or plasma sources, are less than that of the lithography objective.
In another advantageous embodiment, the second optical element with mirror facets is slightly shifted from the plane of secondary light sources, which are formed in the path after the first optical element with mirror facets, in the direction the light is traveling through the illumination system from the light source towards the object plane. Alternatively, the second optical element with mirror facets could be slightly shifted opposite the direction the light is traveling through the illumination system. This achieves a uniform illumination on the mirror facets of the second optical element and, thus, there is a smaller local thermal load. The amount of defocusing is such that the extent of the secondary light sources is smaller than the size of the pupil raster elements, while the width of the non-illuminated edge region is less than 10% of the minimum diameter of the pupil raster elements. A non-illuminated region will occur when the intensity in this region is  less than 10% of the maximum intensity of the secondary light source.
In a preferred embodiment, additional optical elements such as field mirrors are arranged after the mirrors with raster elements and serve to image the pupil plane into the exit pupil of the illumination system, which coincides with the entrance pupil of the projection objective, and to form the ring field in the object plane.
It is especially preferred that the optical elements include grazing-incidence mirrors with an angle of incidence xe2x89xa615xc2x0. In order to minimize the losses of light associated with each reflection, it is advantageous for the number of field mirrors to be kept small. Embodiments with no more than three field mirrors are especially preferred.
Possible light sources for the EUV radiation are laser plasma, plasma or pinch plasma sources, as well as other EUV light sources. Other EUV light sources are, for example, synchrotron radiation sources. Synchrotron radiation is emitted when relativistic electrons are deflected in a magnetic field. The synchrotron radiation is emitted tangentially to the path of the electrons.